1. Field of the Invention
The present invention generally relates to down-hole tools for use in oil and gas well holes and, more particularly, down-hole tools used to locate the position of a stuck point of a drill pipe string in the well hole.
2. Background and Description of the Prior Art
During well drilling operations in the oil and gas well drilling industry, the drill string often becomes stuck in the well. Free point tools have long been used to locate the stuck point of the drill string in the well, so that the drill pipe above the stuck point that is not stuck—i.e., “free”—can be removed to permit further operations to loosen the stuck pipe.
Generally, a free point tool includes a lower portion and an upper portion. When installed within a well casing in a well, the upper and lower portions of a free point tool are configured to detect relative movement of one portion of the tool with respect to the other portion. Traditionally, three basic types of free point tools have been employed. The “magnetic” type utilizes electromagnets to attach the free point tool to the well casing at a portion of the well casing believed to include the stuck point. One example of such a tool is disclosed in U.S. Pat. No. 2,530,309 issued to P. W. Martin for a “Device For Determining Relative Movements Of Parts In Wells.” The “mechanical arm” type causes arms or extensions operated by a mechanical or electrical device to engage the inner surface of the well casing to support the free point tool in the desired position. An example of this type is disclosed in U.S. Pat. No. 5,520,245 issued to James D. Estes, the applicant of the present application, for a “Device To Determine Free Point.”
The third type of free point tool, called the “spring type,” employs two sets of bowed leaf springs, called drag springs, one set associated with an upper portion of the tool and another set associated with a lower portion of the tool, to support the free point tool in the well casing. Typically, three bow springs are used in each set, disposed at 120 degree intervals around the body of the free point tool. An example of the third type is disclosed in U.S. Pat. No. 3,004,427 issued to T. L. Berry for a “Free Point Indicator For Determining The Point At Which Stuck Pipe Is Free In A Well.”
Free point tools are complex devices that must operate in extremely harsh environments where they are subject to wide temperature variations, high pressures, corrosive substances, and the like. Yet, in such conditions, the tool must provide sensitive, reliable measurements of the displacement of the free portion of a well pipe when a tension or torque is applied to the pipe string to cause the displacement. The spring type free point tool has enjoyed substantial commercial success over the years because of it relative simplicity and reliability. However, there are a number of well-known problems with its use.
The drag springs of an exemplary free point tool such as disclosed in the '427 patent must support the entire free point tool assembly—about 39 pounds, including the free point tool (24 lb.), one-half of the slack joint (5 lb.) and the shot rod (10 lb.). To support the tool assembly without slipping, the springs must be set to support nearly twice the weight of the assembly. But this requires extra weight in the form of sinker bars to be added to the weight of the tool assembly, plus the other half of the slack joint, to cause the tool to pass down the well casing easily. In the illustrative prior art free point tool, three sinker bars, at thirty pounds each, are used. The extra weight of these sinker bars is thus about 90 pounds. With each set of drag springs set for about twenty pounds, which is about the maximum that can be effectively used in a well casing environment, the ratio of the weight capacity of the springs to the weight to be supported is approximately one-to-one, making the adjustment of the drag springs relatively critical. If the tension in the drag springs is too high, the tool will not slide down the casing. If the tension is too little, the drag springs will not support the tool and the measurement will not be repeatable. Associated with the critical spring tension adjustment is the high degree of uncertainty that the drag springs will have the correct holding power and that an accurate measurement is made at each desired point in the well casing. Frequently, this uncertainty and the occasional slippage of the springs along the well casing requires that the tool be hauled up, the drag springs reset, and the measurement attempted again.
The sensor assembly in the exemplary prior art tool consists of a variable inductance that must be set electrically to a specified point after the tool has been positioned for taking a measurement. To perform this “reset” the tool must be moved down within the well casing two feet to close the sensor elements, so that the gap between two halves of the variable inductance core is reduced to its minimum value to ensure a repeatable stretch (tension on the drill pipe) measurement. Then, the tool must be raised one foot to center the slackjoint. For torque measurements (application of a right hand torque to the well casing), the inductor is energized with a positive voltage to zero the sensor. Further, a predetermined amount of friction is built in to the variable inductance components so that the setting will be retained after the electrical signal that sets the sensor in position is disconnected prior to taking the measurement. To make the measurement, this friction must be overcome, a factor which affects the accuracy of the measurement. Moreover, this variable inductance varies non-linearly with the displacement of the well casing during the measurement. This characteristic limits the usable sensitivity within a relatively narrow range. Further, in order to ensure adequate repeatability to the measurements, the components of the sensor must be enclosed within a pressure capsule that is filled with oil and equipped with a mechanism to equalize the pressure within the capsule to that within the well casing. Oil seals are required to prevent loss of oil or contamination of the sensor by other fluids in the well casing. The complexity of this sensor design adds weight, reduces reliability and adds to the maintenance expense. The added weight exacerbates the drag spring problems described herein above.
The exemplary prior art spring type of free point tool described above further includes a CCL unit. The CCL unit is a casing collar locator assembly that includes a CCL triggering circuit for igniting a detonating cap that in turn fires the associated string shot explosive to loosen the casing collar joint between the free pipe and the stuck pipe after the stuck point is located. The CCL triggering circuit, which will be called a “CCL” in the description to follow, must have an isolation circuit built in to prevent firing of the detonating cap during a measurement. Typically, this circuit is provided by semiconductor diodes, which limit the effectiveness of the circuit to about 350 degrees Fahrenheit (350 F.) because the diodes lose their blocking characteristics above that temperature. Moreover, the shunting effect of the diodes also affects the free point sensor signal if the same wire is used for both functions. Unfortunately, down-hole temperatures become higher as the depth of the well increases, a circumstance that is more prevalent currently as well drilling extends to deeper levels to access more remote deposits of oil or gas.
One other component of the prior art free point tool described above is a slack joint, which is a sliding joint in the free point tool assembly. This slack joint mechanically decouples the free point tool itself from the entire assembly lowered down the well casing when the desired measurement point is reached. At that point, the free point tool is supported by the drag springs, while the rest of the assembly—the sinker bars—is supported by the wire line cable. The slack joint partially adds to the weight that must be supported by the drag springs.
What is needed is a free point tool that overcomes the problems and disadvantages described herein above. It will be appreciated that the weight of the prior art free point tool that must be supported by the drag springs is a major source of the problems with its use. Further, the prior art sensor design has relatively poor sensitivity, a narrow range of linearity, requires substantial maintenance, requires force to overcome the built-in friction, and requires a relatively complex procedure to reset it for a measurement. Moreover, the prior art tool is ineffective at temperatures above 350 F.